Because of their excellent physical and chemical functions, silicone rubbers are used in a wide variety of applications. At present they are often used as composites with metals and plastics.
One method for promoting adhesion of silicone rubber to various adherends, especially metals, is by applying various primers to the adherends. Since heat resistance is one of characteristics of silicone rubber, a number of primer compositions intended to produce a heat resistant bond were proposed (References 1 and 2: JP-A 61-209269 and JP-A 62-297367).
Usually, primer compositions are previously applied to surfaces of metals, plastics or the like to be bonded, and silicone rubber is then applied thereon and cured via peroxide vulcanization whereby silicone rubber is bonded to metal. Such primer compositions proposed so far include those comprising alkoxysilanes having an unsaturated bond or hydrolytic condensates thereof such as vinyltrimethoxysilane, vinyltri(2-methoxyethoxy)silane, or 3-methacryloxypropyl-trimethoxysilane, or co-hydrolytic siloxanes, titanates, and metal salts of organic fatty acids (References 3 and 4: JP-B 62-17622 and JP-B 62-17623), and a primer composition comprising such a siloxane composition, a metal salt of an organic fatty acid, and an organosilicon compound having SiH group (Reference 5: JP-B 61-2107). In Reference 6: JP-A 60-115661, an alkoxy-containing polysiloxane and a platinum base catalyst are compounded in such a primer composition.
The primer compositions of References 1 to 5 are effective in improving adhesion, but the absolute magnitude of bond strength is not sufficient. Also, none of them achieve the effect of improving adhesion to noble metals which are least corrodible. The primer composition of Reference 6 will gel within about one day, losing its primer function. Typically the primer composition is applied to a noble metal part (such as gold, silver or copper) which is least prone to oxidation or degradation in a hot atmosphere. Then a millable silicone rubber having organohydrogenpolysiloxane compounded therein is vulcanized and bonded to the primed metal part within a time when the primer composition maintains its primer function. Even in this event, sufficient adhesion is not necessarily achieved.
References 7 and 8: JP-A 09-71664 and JP-A 2007-119752 disclose to use triazinethiol derivatives for improving adhesion to difficult-to-bond metal substrates. In Reference 7, once a molded resin is metallized with a nickel or copper-containing metal, a rubber composition containing an alkali metal salt of triazine trithiol is crosslinked in contact with the metallization, thereby achieving crosslink bonding between the metallization and the rubber so that the rubber is bonded to the molded resin via the metallization. Since the triazine derivative is compounded in rubber, it is not believed that a high proportion of the crosslink-contributing component is available at the bonding surface. This method achieves an adhesion improving effect which is insufficient as compared with the direct treatment of adherend surfaces with primers.
The method of Reference 8 involves treating a molded body of hydroxyl-containing resin with a solution containing an alkali metal salt of an alkoxysilyl-containing triazine dithiol as a silane coupling agent and bonding rubber to the molded resin body via the treated surface. However, the silane coupling agent used in this method is poorly soluble and is present in a low concentration so that the treatment of the resin is less efficient, failing to achieve consistent adhesion. Because of a silane coupling agent, the substrate to which rubber can be bonded is limited to those materials having on their surface OH radicals or functional radicals capable of chemical bonding with alkoxysilyl radicals. The agent is not applicable to gold, silver, copper and similar metals which are least reactive with alkoxysilyl radicals. In addition, triazinethiol moieties incorporated in crosslinks with rubber are less reactive.